1. Field of the Invention
This invention relates to acidizing of subterranean formations surrounding oil wells, gas wells and similar boreholes. In one aspect, it relates to an improved fluid loss composition for use in acid fracturing.
2. Description of the Prior Art
Acid fracturing is a widely used technique for stimulating the production of subterranean carbonate formations surrounding oil wells, gas wells, and similar boreholes. Carbonate formations include limestones, dolomites or other reservoir rocks that contain substantial amounts of calcareous material. Acid fracturing of these formations normally involves the injection of an aqueous acid solution into the wellbore at a rate and pressure sufficiently high to fracture the surrounding formation. Stimulation is achieved by the acid etching the fracture walls, thereby providing highly conductive channels when the fracture closes. Tests have shown that the degree of stimulation afforded by acid fracturing is strongly dependent upon the extent of acid penetration in the fracture. One technique for providing deep acid penetration involves the use of gelled acids. The high viscosity of the gelled acid retards the rate of acid transport to the reaction site at the fracture wall and thus permits unspent acid to penetrate deeply into the fracture. Use of finely divided fluid loss additives have also been proposed to reduce the rate of acid transport to the reaction site. These finely divided materials retard fluid leakoff into the formation by building up a filter cake on the fracture wall. Laboratory tests, however, have shown that the effectiveness of most commercial fluid loss additives in attaining deeper acid penetration is only marginal. The reason for the poor performance of these materials is believed to be due to the tendency of the acid to quickly form wormholes in the fracture wall and thereby eliminate the effect of the filter cake provided by the fluid loss particles.